1. Field of the Invention
The present invention relates to a transmission power control system that controls transmission power of a radio signal transmitted by a small base station, a base station, and a transmission power control method.
2. Description of Related Art
Conventionally, in a mobile communication system, it is necessary to secure a service quality according to content of a communication request generated irregularly from a mobile station. Therefore, when a mobile communication carrier installs the base station, it is general to design a radio parameter, such as an installation location or a configuration of a base station, based on estimating the size of a service area or the traffic generated within the service area, and a transmission power of a common control channel in a downlink direction transmitted from a base station, by means of simulation (for example, Mori, et al., IEICE General Conference 2001, B-5-34 “W-CDMA Area Evaluation Experiment Using Reception Quality Measurement System” March 2001) or the like.
In recent years, in addition to a general base station for a public communication installed by a mobile communication carrier, the number of small base stations installed at home by a mobile station user (for example, Home NodeBs) such as femto cell increases (for example, see 3GPP TS 22.220, Service requirements for Home NodeBs (UMTS) and Home eNodeBs (LTE)). When the mobile communication carrier designs the aforementioned radio parameter, the installation location of such a small base station also needs to be considered.
However, the small base station can be freely installed by the mobile station user, and thus, it is not realistic to design the radio parameter based on simulation in which installation locations of all the small base stations, for example, are considered. Thus, there is a need of Self Configuration by which a small base station, etc., autonomously set the radio parameter.
For example, it is possible to consider the following method, i.e., the small base station measures a reception level of a common control channel (in W-CDMA system, Common Pilot Channel (CPICH)) transmitted by a peripheral base station (a small base station or a general base station), and based on the measured reception level, a transmission level of a common control channel transmitted by the small base station is determined.
However, the aforementioned self configuration has the following problem. That is, when the small base station determines the transmission level of the common control channel transmitted by the small base station based on the reception level of the common control channel transmitted by the peripheral base station, the transmission level of the common control channel of the small base station needs to increase in order to reduce a quality deterioration at the mobile station caused by the common control channel of the peripheral base station.
There is a problem that if such a process is repeated in the peripheral base station, then the transmission level of the common control channel transmitted by the small base station or peripheral base station increases in the end, and therefore, an interference of a radio signal between the base stations can rather increase.